1. Field of the Invention
The present invention relates to a method of and an apparatus for friction stir welding for joining an abutment portion by moving a rotating probe relatively along the abutment portion while pressing the probe against one surface of the abutment portion at which an end of a first plate member and an end of a second plate member are butted together.
2. Description of the Related Art
The friction stir welding (hereinafter referred to as “FSW” as well) is known, in which two workpieces are joined in solid phase by utilizing the frictional heat which is generated when a rotating probe is inserted into the workpieces. When FSW is carried out, the joined portion of the workpiece can maintain a strength which is about 80% of that of the base material, and it is also possible to prevent crystals from becoming coarse.
If the welding is performed by means of the conventional spot welding or the conventional electron beam welding by using an aluminum material as the workpiece, for example, some excessive heat is applied to the aluminum material. Therefore, the strength may be decreased due to deterioration of the material and/or change of crystals into coarse microstructure.
In contrast, the joining by FSW is performed at about 500° C., the material is prevented from the deterioration which would be otherwise caused by the heat, even when a metal material such as the aluminum material having a relatively low melting point (about 600° to 660° C. in the case of the aluminum material) is used. Therefore, FSW is also applicable to other materials such as magnesium, titanium, and polymer as well as the aluminum material.
As an example of the application of FSW to the aluminum material, an aluminum frame of a large member such as an electric train may be exemplified. In the case of such a large member, the joining strength is important, and hence the thickness of the aluminum material is usually not less than 5 mm. On the other hand, if light weight is desired as well as the improvement of strength in the case of gas turbine engine members or the like, it is difficult to increase the thickness of each component. For this reason, for example, a thin plate aluminum material of about 1.2 mm thickness is used to construct an outer frame of the gas turbine engine.
However, when an abutment portion, at which both ends of thin plate-shaped aluminum materials are butted together, is joined by means of FSW to form a cylindrical member having a relatively large diameter, it is difficult to obtain a satisfactory circularity or roundness because the aluminum material is thin.
Further, when an abutment portion, at which ends of two cylindrical members are butted together, is joined by FSW, the circumferential lengths of the respective ends may not be the same. If the cylindrical members are joined in this state by FSW, then the difference in phase appears at the final position of the joined portion, and for example, the shape is deformed in a wavy form, resulting in wrinkles or corrugations.
Furthermore, when the aluminum material is thin plate, unjoined portion tends to appear when the abutment portion is joined by FSW. Specifically, as shown in FIG. 19, when a rotating probe P is inserted into an abutment portion T where aluminum materials W1, W2 are butted together, an unjoined portion U may appear at the abutment portion T due to shortage of friction stir caused by the failure of arrival of the probe tip Pa.
Moreover, the circumferential speed of the probe P is slowed down as the position approaches the probe tip Pa. The area S, which is subjected to the friction stir, is restricted or reduced in the vicinity of the probe tip Pa due to the decrease in circumferential speed. As a result, the frictional heat is insufficient, and the unjoined portion U appears. Because the unjoined portion U may be a fracture origin, structural reliability is deteriorated.
Additionally, the pressing force of 1 t to 2 t is applied to the thin plate aluminum materials W1, W2 during the joining by FSW. Therefore, some irregularity may appear at the joined portion as a result of FSW, if the aluminum materials W1, W2 are not retained reliably.
In view of the above, Japanese Laid-Open Patent Publication No. 11-226759 describes a method of joining aluminum members as shown in FIG. 18, for example. A backing member 3 of the same material as that of hollow tubes (plate members) 1a, 1b is arranged at the inside of an abutment portion 2 of the hollow tubes 1a, 1b made of aluminum. A predetermined interstice or clearance C is formed between the backing member 3 and the inner surface of the abutment portion 2.
In this arrangement, a probe tip 4a of a probe 4 is inserted into the abutment portion 2 and the backing member 3 while rotating the probe 4 at a high speed, and the probe 4 is moved along the abutment portion 2. Accordingly, the entire circumference of the circumscribing abutment portion 2 is welded by the friction stir welding.
In the case of Japanese Laid-Open Patent Publication No. 11-226759, the backing member 3 is provided as a reinforcing member of the hollow tubes 1a, 1b, and the reinforcing member is joined to the inner surfaces of the hollow tubes 1a, 1b. However, the hollow tubes 1a, 1b, to which the backing member 3 is joined as described above, cannot be used, as an outer frame of a gas turbine engine, for example. Therefore, it is necessary to exfoliate the backing member 3 from the inner surfaces of the hollow tubes 1a, 1bafter joining the hollow tubes 1a, 1b. In particular, when the thickness of the hollow member 1a, 1b is small, strain or distortion appears due to the exfoliation treatment of the backing member 3. Further, the backing member 3 may be broken during the exfoliation, and cannot be used repeatedly if broken. Therefore, it is not economical.
Another method is described in Japanese Laid-Open Patent Publication No. 10-225780 as a method of producing an abutment joint. In this production method, as shown in FIG. 20, end surfaces of joining members 1a, 1b made of aluminum are butted together to provide an abutment portion 2. A backing member 3 made of aluminum is arranged on the abutment portion 2. A recess 3a, which has a circular arc-shaped cross section, is formed on a surface of the backing member 3.
In this arrangement, a rotor 4 and a probe 5 are rotated, and the probe 5 is inserted into the abutment portion 2 of the joining members 1a, 1b. Accordingly, the abutment portion 2 of the joining members 1a, 1b is deformed to protrude on the back surface along the recess 3a of the backing member 3. When the probe 5 is moved along the abutment portion 2 with the probe 5 inserted into the abutment portion 2, the abutment portion 2 is subjected to the friction stir welding over the entire length of the abutment portion 2. Subsequently, a deformed portion 6, which is deformed to protrude toward the recess 3a of the backing member 3, is cut off by using, for example, a milling machine to obtain a flat surface.
However, in the case of the technique described in Japanese Laid-Open Patent Publication No. 10-225780, it is necessary to insert the probe 5 to the back surface of the abutment portion 2, in order that the abutment portion 2 is deformed on the back surface side to protrude toward the recess 3a of the backing member 3. For this reason, especially when each of the joining members 1a, 1b has a small thickness, breakage may occur in the joining members 1a, 1b. 
Further, it is necessary to exfoliate the backing member 3 from the back surface of the abutment portion 2 after the joining process. Therefore, when the joining members 1a, 1b are thin as described above, residual strain appears as a result of the exfoliation treatment of the backing member 3.
Also in this case, the backing member 3 may be broken during exfoliation, and cannot be used repeatedly if broken. Therefore, it is not economical.